It is known in the art to dose a reagent, such as urea solution, into the exhaust system of an internal combustion engine in order to enable a selective catalytic reduction (SCR) catalyst to reduce oxides of nitrogen (NOx) in the exhaust gas stream. The dosing of reagent is typically performed using a fluid dosing pump or fluid doser.
An example of a known fluid dosing pump is described in the Applicant's published European Patent No. 1878920. Such a dosing pump is usually mounted to a hot exhaust system and, accordingly, relies on a combination of insulation and the cooling effect provided by the reagent fluid being pumped through it in order to prevent overheating.
The exhaust systems of modern diesel engines are typically fitted with diesel particulate filters (DPF) to remove soot from the exhaust gas stream. A DPF requires periodic “regeneration”, which involves raising the temperature of the exhaust gases to a higher than normal temperature in order to burn off the soot trapped in the DPF. Occasionally, an “extreme regeneration” is required, during which the exhaust gases are raised to a temperature even greater than during the normal regeneration process.
During an “extreme regeneration” event, the high exhaust gas temperatures tend to release ammonia stored in the SCR catalyst, which is able to reduce all the oxides of nitrogen present. Accordingly, in such circumstances it is not desirable to dose reagent using the dosing pump because the reagent is not required for SCR and is wasted. However, by reducing or stopping dosing, the dosing pump may be adversely affected due to the extreme exhaust gas temperatures combined with the reduced cooling flow of reagent through the pump. It has been determined from engine testing under such conditions that the fluid inside the main pump body can boil, preventing the pump from dosing correctly.
It is an object of the present invention to provide a fluid dosing pump which substantially overcomes or mitigates the aforementioned problem.